1. Field of the Invention
This invention relates to an exhaust emission monitoring system and more particularly relates to a spectroscopic absorption remote sensing instrument for multi-component analysis of exhaust plumes emitted by in-use vehicles.
2. Background Information
Emissions from mobile sources are well known to play a central role in urban air pollution (photochemical smog formation, violation of carbon monoxide (CO) and ozone (O3) standards, and aerosol formation). In 1994, the U.S. Environmental Protection Agency (US EPA) estimated that, for the previous year, U.S. on-road vehicles contributed 62%, 32%, and 26% of all CO, nitrogen oxide (NOx), and volatile organic compound emissions, respectively. (xe2x80x9cNational Air Pollution Trends, 1900-1993,xe2x80x9d United States Environmental Protection Agency, Office of Air Quality Planning and Standards, 1994).
It is well known that numerous chemical species can be monitored non-invasively in ambient air with a high degree of precision (xe2x80x9cAir Monitoring by Spectroscopic Techniquesxe2x80x9d, Ed. Sigrist, M. W., John Wiley and Sons, New York). Remote sensing of exhaust from light duty motor vehicles has provided a wealth of useful information with respect to CO and total hydrocarbon (HC) emissions. Data collected from these investigations indicate that approximately half of CO and HC emissions from in-use vehicles are generated by less than 10% of the fleet.
Over the past 3 years NO emissions have been reported to follow similar trends. Moreover, remote sensing data suggests that fleet dynamometer testing significantly underestimates tailpipe emissions, and contributes to errors in model predictions (e.g., U.S. EPA""s MOBILE4). A knowledge of the chemical composition of the exhaust plume emitted by on-road vehicles on a car-by-car basis therefore is essential when developing effective pollution abatement strategies.
Most existing remote sensing studies have relied on non-dispersive infrared (NDIR) and non-dispersive ultraviolet (NDUV) spectroscopy. This research resulted in several patented inventions (C. V. Swanson, Jr., U.S. Pat. No. 4,924,095, issued May 8, 1990; G. Bishop and D. H. Stedman, U.S. Pat. No. 5,210,702, issued May 11, 1993; L. H. Rubin and M. D. Jack, U.S. Pat. No. 5,418,366, issued May 23, 1995; M. D. Jack, U.S. Pat. No. 5,591,975, issued Jan. 7, 1997; M. D. Jack et al, U.S. Pat. No. 5,831,267, issued Nov. 3, 1998). They are based on NDIR analyzers that rely on a plurality of detectors, one for each monitored gas as well as one reference detector, to make their measurements. In one case (Bishop et al.) an NDUV channel is added to measure NO. They share similar optical designs, such as a rotating polygonal mirror to channel the unfiltered radiation sequentially to each detector, and use narrow bandpass optical filers to isolate the spectral window that matches the absorption feature of the gas of interest.
Optical designs for the remote sensing of vehicle exhaust have also been patented separately (M. E. Sullivan et al, U.S. Pat. No. 5,563,420, issued Oct. 8, 1996; J. Didomenico et al, U.S. Pat. No. 5,644,133, issued Jul. 1, 1997). While these analytical techniques provide excellent data for CO and CO2, accurate HC data has been difficult to acquire due to modest sensitivity, typically 500 parts per million (ppm, 1 ppm=1 part in 106 by volume or moles) detection limit (3"sgr"), and water interference. The sensitivity to NO is even poorer (300 ppm precision, 1"sgr"), limiting the instrument to the identification of gross polluters or fleet evaluations.
Bishop et al. later enhanced their invention by replacing the NDUV channel with a dispersive spectrometer using a photodiode array detector (G. Bishop et al, U.S. Pat. No. 5,401,967, issued Mar. 28, 1995; G. Bishop et al, U.S. Pat. No. 5,498,872, issued Mar. 12, 1996). However, the first reported use of a dispersive spectrometer using an array detector to remotely monitor the NO UV band in vehicle exhaust dates back to 1984. (Pitts, J. N.; Biermann, H. W.; Winer, A. M.; Tuazon, E. C. Atmos. Environ. 1984, 18, 847-854).
More recently, tunable infrared diode laser absorption (TIDLA) spectrometers have been utilized in a remote sensing configuration to measure NO in vehicle exhaust with greater sensitivity and selectivity. More patents have resulted from these inventions (D. D. Nelson et al, U.S. Pat. No. 5,877,862, issued Mar. 2, 1999). Unfortunately, these TDLA spectrometers are impractical for field use as they require cryogenic cooling to operate in the mid-IR (where most of the strong bands are located). They also need skilled operators and can be prohibitively expensive for multi-component applications. Fourier transform infrared (FTIR) spectrometers have become very popular for open path monitoring, but have found limited application in the remote sensing of auto exhaust. This is partially due to low signal-to-noise ratios from the short averaging times (0.5-1.0s). Additionally, such systems are often too delicate for field use; sturdier and faster spectrometers are available, but can be expensive.
A rugged, low-cost alternative to existing remote sensors is needed to measure criteria pollutants (CO, HC, NO), as well as CO2, with equal or increased precision. The instrument should also be capable of measuring other compounds of importance to tropospheric photochemistry. For instance, formaldehyde (HCHO) and acetaldehyde (CH3CHO) are key to the photolytic generation of hydroperoxyl and acylperoxy radicals; nitrous acid (HONO) is an important source of hydroxyl radicals; nitrogen dioxide (NO2) affords ozone upon photolysis and reacts with hydroxyl radicals to yield nitric acid; aromatic hydrocarbons (e.g., benzene, toluene, xylene) are important reaction sinks for hydroxyl radicals, often affording secondary organic aerosols. Ammonia (NH3) is known to be emitted by vehicles equipped with three-way catalysts operating under fuel-rich conditions. NH3 emissions play a key role in the production of fine particulate matter. Finally, a useful remote sensor should store sufficient spectral information in the xe2x80x9csnapshotxe2x80x9d of the exhaust plume to enable quantification of xe2x80x9cunknown speciesxe2x80x9d at a later date.
These challenges can be met with the disclosed invention by using a novel optical design to allow dispersive IR and UV-vis spectroscopy in combination with powerful chemometric techniques.
The principal purpose of the disclosed invention consists in the quantitative analysis of gas-phase components and particulates in the exhaust plume emitted by moving vehicles. This measurement is made non-invasively by using novel remote sensing technology.
The present invention is unique in its optical design and by virtue of the fact that dispersive IR and UV-vis spectrometers are used to make the measurements. The use of wavelength-resolved data both in the IR and UV-vis has not been reported previously to make remote measurements of pollutant levels in the exhaust plume of in-use vehicles.
The instrument is capable of measuring numerous pollutants emitted by in-use vehicles, including, but not limited to, CO, CO2, HC (as propane), NO, and dust.
The instrument has the capability of measuring numerous other pollutants, such as NH3, sulfur dioxide (SO2), CH3CHO, HONO, NO2, N2O, toluene, benzene, xylene, benzaldehyde. Addition of these measurement channels does not necessitate any hardware modifications and only requires minor adjustment to the analyzer software configuration file.
The analyzer is able to achieve the above measurements by using a pair of collimated infrared (IR) and ultraviolet (UV) beams emitted at right angles across a roadway. There, the beams are collected by an optical device, such as a spherical mirror, and returned to the analyzer. Thus, two, or multiples of two (i.e., 2, 4, 6, 8, etc.), optical passes are made across the roadway traveled by moving vehicles. The radiation is analyzed by one or more dispersive UV and one or more dispersive IR spectrometers.
The UV and IR spectrometers generate spectra at high frequencies (e.g., 100 Hz) and these spectra span a wide spectral window (e.g., 200 nm in the UV and 2000 cmxe2x88x921 in the IR). Characteristic absorption signatures of numerous gases occur in these spectral windows, such as CO, CO2, and aliphatic HC (as propane) in the IR, and NO in the UV, and dust scattering in the visible. One or more pattern recognition algorithms match a spectral database, containing reference spectra of the gases of interest, to the spectra of the vehicle exhaust and, herewith, calculate the concentration of the analytes of interest in the exhaust plume.
The concentrations of the analytes of interest are output to a storage device in the analyzer and to a data acquisition system by means of a standard digital data communication protocols (e.g., RS-232). A set of electronic triggers also are output with the data to validate the measurement, and indicate pass/fail status of the vehicle with respect to pollution emission regulations or standards.
The above and other objects, advantages, and novel features of the invention will be more fully understood from the following detailed description and the accompanying drawings, in which: